In a manufacturing process of semiconductor devices, a plurality of wafers which become substrates for the semiconductor devices are housed in a cassette 20 comprising a plurality of shelves as shown in FIG. 1 and transported in a clean room. Recently, in order to highly prevent foreign materials causing short circuit of a fine electric circuit on wafers from adhering to the wafers, the wafers are transported while being housed in a hermetically sealed clean container. These wafers are taken out from the clean container in a highly clean booth, and then provided with various treatments such as inspection and processing. After that, when a wafer is returned to the cassette or the clean container by using a transport device such as a robot, it is necessary to align the wafer center point with the center position of a finger of the robot before starting housing operation into the cassette etc., in order to prevent failures, such as damaging or dropping the wafer due to contact between the wafer and the cassette wall.
In the processes requiring information on the wafer position such as patterning, deposition, and chemical vapor deposition processes and various inspections, consistently and accurately positioning a notch part of the wafer outer periphery edge such as an orientation flat (hereinafter referred to as “OF”) and a notch, and a wafer center point in given positions is an important run-up operation. Therefore, before shifting to the foregoing processes, it is necessary to lay the wafer on a wafer positioning device generally called an aligner, detect the position of the wafer center point and the orientation of the notch part, accurately move the wafer to the correct position, and then hand the wafer over to various processing devices, various inspection devices, robots or the like.
The wafer positioning device generally comprises a small circular table called a spindle having a size to allow the wafer to lie thereon as a wafer seat on the rotational shaft. The center point position and the notch part position of the circular wafer are calculated by rotating the spindle along with the wafer placed thereon by rotation of the rotational shaft; and detecting an eccentric radius from the rotational shaft or the rotation axis of the spindle to the wafer outer periphery edge and a rotation angle by using a line sensor and an angle sensor such as an encoder. After that, the wafer is moved in the extending direction of an X axis and a Y axis by the distance calculated as above, and rotated by the angle calculated as above. Then, the wafer is accurately positioned in a given position and a given direction. Such a positioning device has been conventionally suggested in, for example, Japanese Patent Application Opened No. S59(1984)-147444, Japanese Patent Application Opened No. H05(1993)-343501, and Japanese Patent Application Opened No. H06(1994)-224285.
For example, a device described in Japanese Patent Application Opened No. H05(1993)-343501 comprise drive means to move a wafer seat in the direction of the X, Y, and Z axes and rotate the wafer seat. In this device, data obtained from outer periphery edge detection means by rotating a wafer is A/D-converted, the converted data is directly transferred to a storage circuit by DMA data transmission means. Calculation time is thereby shortened. Further, in this device, as shown in FIG. 19 an eccentric amount Le and an eccentric angle 60 are calculated from wafer outer periphery signals Lx1, Lx2, Lx3, and Lx4 at one end point 0x1 of the notch part and other three points 0x2, 0x3, 0x4 on the outer periphery edge, which are distanced from 0x1 sequentially by each 90° about the rotation axis B, by processor means by using the following expressions:Le=½·{(Lx3−Lx1)2+(Lx4−Lx2)2}1/2θ0=tan−1−{(Lx3−Lx1)/(Lx4−Lx2)}
Here, the eccentric angle θ0 of one end of the notch part is determined by storing and comparing a ratio between fine angle Δθx obtained for the foregoing 4 points and corresponding eccentric radii ΔLx and the immediately preceding ratio ΔLx−1/Δθx−1 and obtaining a point wherein the ratio becomes a fixed value or more and ΔLx becomes the maximum while the wafer rotates 360°. In result, first, the wafer center point is moved to a given position, the wafer is retaken up, and the rotation axis is aligned with the wafer center point. Next, the wafer is rotated until the notch part is moved to a given position, and then stopped. Consequently, shortening the positioning time for all processes from conventional 8 to 10 sec. down to 3 to 4 sec. is attained.
In Japanese Patent Application Opened No. H06(1994)-224285, a wafer positioning device having a machine construction similar to the foregoing is used. As shown in FIG. 20, a sum of four eccentric radii to four points at which two orthogonal straight lines passing through the rotation axis B intersect the wafer outer periphery edge is approximated by about twice the diameter. When the perpendicular straight lines are rotated by every certain angle 0 (=about 10°) up to 360°, and eccentric radii an, bn, cn, and dn are measured, results of Ln=an+bn+cn+dn are almost constant over the angle 360°. However, since the result of Ln=an+bn+cn+dn for the notch part is the minimum, the notch part is thereby obtained. Here, a deviance between the rotation axis B and the wafer center point is obtained from the four eccentric radii derived from the perpendicular straight lines passing through the wafer center point.
Incidentally, in recent semiconductor processes, wafer sizes have become large, e.g. a wafer diameter of 300 mm in order to improve productivity, and wafer weight has become large, twice the conventional weight. Despite the increased wafer sizes, the wafer positioning device is also required to have a high speed and high precision.
However, in any of the foregoing conventional devices, data is collected over 360° every fine angle for four points where the straight lines perpendicular to each other passing through the rotation axis intersect the wafer outer periphery edge. Therefore it takes time to collect and calculate lots of data. Further, since the wafer has to be rotated at least one revolution, its rotation time is also required. Furthermore, in the foregoing conventional devices, the wafer has to be retaken up between wafer center point alignment movement and angle alignment for the notch part. Therefore, entire duration is not short enough.
In any of the foregoing conventional devices, respective split angles of 360° are rough. Further, when the notch part is detected, the minimum point, which is the lowest point dropped from a smooth line drawn in a diagram showing a relation between eccentric radii and rotational angles, for example, shown in FIG. 9 is regarded as a center of the notch part, which is not an original center of the notch part. Therefore, accuracy is lacked, and there is a problem that the wafer positioning precision is low.
In order to solve the issues of the high speed and the high precision, inventors of the present application have devoted themselves to research on a positioning method which does not require the time-consuming operation processes, such as retaking up the wafer and stop or re-rotation in the middle of rotation, and the process for processing lots of data, causing load on calculation means such as a calculator. In result, a method which is significantly enables high precision and shortening time compared to the conventional methods has been found, and a wafer positioning device and a wafer process system have also been developed.
Furthermore, in order to calculate a position of the wafer center point by rotating the wafer as described previously, it is necessary to accurately position the rotation axis of the spindle at the point where a straight line in the direction of spindle movement (generally an X axis of the positioning device) and a straight line which passes through the center part of the line sensor and extends in the extending direction thereof (generally a straight line parallel to a Y axis of the positioning device) cross perpendicular to each other when operation is started. Otherwise, a correct eccentric radius from the rotation axis of the spindle to the wafer outer periphery edge cannot be measured.
Therefore, in any of the conventional positioning devices suggested in the foregoing documents, a position relation between the line sensor and the rotation axis of the spindle is mechanically fixed as above. These conventional devices are based on the assumption that such a position relation does not change over long periods.
However, actually, when the positioning device is assembled, when the positioning device is installed at a semiconductor factory, when some object hits the line sensor by accident, causing a position deviance, or when the line sensor is replaced, subtle arrangement of the sensor position is required. Skilled engineers spend considerable time on the maintenance operation for such an adjustment. Further, when positioning operation is performed while the sensor position remains deviant, if a certain allowance is exceeded, alarm is given and this operation is stopped. Meanwhile, if eccentricity is within an allowance, the wafer positioning operation continues with the eccentricity remained, often resulting in a low-precision positioning.
In light of the foregoing actual conditions, the inventors of the present application have developed a wafer seat rotation axis positioning method in which the wafer positioning device automatically and accurately positions the rotation axis on the line sensor center line perpendicular to an X axis before starting wafer position detection operation, so that a wafer positioning precision based on subsequent wafer position detection operation is improved and that maintenance free of the wafer positioning device is realized.